pdfAddRegion

The XpdfViewer library can display a set of highlighted regions on the
currently open PDF file. Once a highlighted region is created, the
user can manipulate it by selecting it (clicking on it) and then
moving its edges to resize it. As far as the XpdfViewer library is
concerned, highlighted regions are simply rectangles drawn in some
specified color. You can use regions for various purposes, e.g.,
allowing the user to mark parts of a PDF file to be extracted later.